Reducing the temperature in the seal chamber offers many benefits to the performance and reliability of a mechanical seal operating in hot service. This is one of the most effective ways to increase the vapor pressure margin and prevent the pumped fluid from flashing in the seal chamber or at the interface of the mechanical seal’s faces. Additionally, lowering the seal chamber temperature also increases the fluid’s viscosity, providing a more stable fluid film at the interface of the seal faces.
One method of achieving a reduction in temperature is to circulate fluid from the seal chamber through a heat exchanger and return the cooled fluid back into the seal chamber. The heat exchanger is often referred to as a “seal cooler” since it is not part of the process, but just an auxiliary system component. This piping arrangement is known as an API Plan 23. When installed, operated and maintained correctly, a Plan 23 is one of the most effective methods of lowering the seal chamber temperature.
Fluid is circulated through the heat exchanger by a pumping ring incorporated into the mechanical seal’s design. The pumping ring, typically a slotted wheel or helical scroll, is spinning with the pump shaft and functions as a miniature pump within the seal chamber. In comparison to the main impeller on the pump shaft, the pumping ring only generates an extremely small fraction of pressure head and flow. Thus, it is of critical importance that the design, selection and installation of the flow circuit is optimized to provide the least resistance to flow, thereby maximizing the circulation rate and the ability of heat energy to be transferred from the seal chamber to the heat exchanger.
Optimizing the Flow Circuit
There are three main elements to the flow circuit that can be optimized:
- The heat exchanger
- The interconnecting tubing between the heat exchanger and seal chamber
- The entry and exit ports in the seal chamber (or mechanical seal) and their position relative to the pumping ring
The heat exchanger needs to be of a suitable size to dissipate the heat load placed on it while offering minimal resistance to flow. Water-cooled shell and tube heat exchangers meet these requirements and are often the first choice for a heat exchanger design. Plate-style heat exchangers, although compact and with large heat transfer rates, should be avoided as their resistance to flow is high. Air-cooled heat exchangers can be used in water constrained installations. However careful design and selection is required to meet the cooling capacity needed while not being excessively large.
The preferred method to connect the heat exchanger to the seal chamber is using drawn tubing (where codes and standards allow). The diameter should match that of the heat exchanger coil. If in doubt, a larger size should be selected. Note that excessively large sizes will not yield positive results and may be detrimental to lowering the flow resistance of the circuit. To minimize the resistance to flow in the tubing, valves should be avoided. If they are required, they should be full-ported gate type or locking ¼-turn ball valves. The number of bends in the tubing should also be minimized, only using long radius bends and avoiding the use of short 90-degree fittings. The overall length of the tubing run should be kept to a minimum.
Where sufficient space exists in the seal chamber, the most efficient pumping ring designs and the flush ports delivering fluid to and from the pumping ring are designed as an integral part of the mechanical seal and its housing (see Figure 1). This allows the seal manufacturer to optimize the location of the flush ports to gain the maximum pressure and flow from the pumping ring. It also enables the correct cooling flow path through the mechanical seal, ensuring cool fluid is delivered to the mechanical seal faces.
For pumps with horizontal shafts, the flush-out port should be located at the top of the seal housing to enable the seal chamber to be vented of any trapped gases, and the flush-in port located at or below the shaft centerline. Vertical shafts should have the flush-out port at the uppermost point in the seal chamber to achieve complete venting. This typically necessitates the use of an axial flow pumping ring with the flush-in port located below the pumping ring.
Since the Plan 23 flow circuit is not truly a closed loop, loss of cooled fluid in the seal chamber occurs as fluid enters and exits the seal chamber throat. This mixing of hot fluid from the pump and cooled fluid in the seal chamber can be minimized by the addition of a close clearance seal chamber throat bushing. A fixed bushing is suitable in most cases. However, tighter clearances can be achieved with the use of a floating bushing. It is a good practice to run floating bushings against a renewable surface, such as a sleeve, rather than against the pump’s bare shaft. This bushing can be integrated into the design of the mechanical seal or installed as a separate item into the throat of the seal chamber. When installed as a separate item, the bushing should be renewed with each mechanical seal change to minimize mixing of hot and cold fluids in the seal chamber. The retention method of the bushing must consider the temperature differences of the pump casing, which will be close to process temperature, and the bushing parts that are closer to the much lower seal temperature.